The present invention relates to a ring groove for an oil ring with a coil expander of a piston.
FIG. 1 shows an enlarged cross-sectional view of an illustrative ring groove of prior art. As shown in FIG. 1, conventional ring grooves 1 of a piston 20 for oil rings 10 each having a coil expander 6, have been of a rectangular cross-section either with square inner corners, or rounded inner corners as proposed in Japanese Utility Model Laid-Open Publication (JP-U) 47-38204 (1972). Such a ring groove 1 of a rectangular cross-section, includes a radially inward-most, bottom surface 2, an upper inner surface 3 and a lower inner surface 4, both orthogonal to the axial direction of the piston 20. The ring groove 1 has a two-piece oil ring 10 mounted or inserted therein, where the oil ring 10 includes a ring body 6 and a coil expander 6 assembled together, with the axial width of the ring groove 1 set to be slightly greater than the width of the ring body 5, specifically, for example, by 0.04-0.06 mm.
When used in this text, xe2x80x9caxiarxe2x80x9d and xe2x80x9cradialxe2x80x9d respectively mean association with the axial and radial directions of a piston wherein the ring groove is formed, and xe2x80x9cinwardxe2x80x9d means a sense toward the center of the piston in the radial direction of the piston.
The bottom surface 2 of the ring groove 1 must not be in contact with the radially inward end of the coil expander 6 even when the piston 20 does an oscillating motion when actually working involving thermal expansion. Because of this, the bottom surface 2 was set to be located radially inwardly apart from the radially inward end of the coil expander 6 by a predetermined distance, e.g., around {fraction (1/200)}, of the inner diameter of a cylinder 7 engaged with the piston when the outer circumferential surface of the ring body 5 is in contact with the inner circumferential surface of the cylinder 7.
An opening 9 of each of oil drain holes 8 to drain the oil within the ring groove 1 to the oil pan, was provided in a position lower than the center of the axial width of the ring groove 1 on the bottom surface 2, or on the lower inner surface 4 of the ring groove 1.
The coil expander 6 of a two-piece oil ring projects inward from the inward or rear end of the ring body 5 by a distance equivalent to nearly a half of the outer diameter of the coil expander 6. Therefore, the cavity space behind the rear end of the ring body 5 of the oil ring 10 in the ring groove is relatively large. This cavity space has become far greater especially when the ring body has been made thinner to improve the conformability of the oil ring to the cylinder.
FIG. 2 is an illustrative graph showing the axial behavior, frictional force and inertia force observed with an actually working oil ring, and FIG. 3 is a schematic drawing showing the behavior of oil within a ring groove, showing a prior art example in (a), (b) and (c), and an embodiment case in (d), (e) and (f). As shown in FIG. 2, when actually working, an oil ring in a ring groove is affected by frictional force with the cylinder, inertia force caused by the piston motion, and gas pressure caused by combustion. In the exhaust stroke, even when the inertia force acts upward, the oil ring 10 stays on the lower side of the ring groove 1 during a certain period due to the frictional force, whereby a gap is caused above the upper surface of the oil ring 10, as shown in FIG. 3(a). At that time, the oil in the cavity space behind the rear end of the ring body in the ring groove, gets in the gap above the upper surface of the oil ring 10. Subsequently, when the inertia force overcomes the frictional force, and the oil ring 10 thereby moves toward the upper side of the ring groove 1, the oil that got in the gap above the upper surface of the oil ring 10 is pushed out toward the outer circumferential surface of the piston, as shown in FIG. 3(b), Subsequently, as shown in FIG. 3(c), the oil rises from the ring groove 1 by means of the inertia force, and moves toward the combustion chamber to be burned and disappear there. Thus, if the cavity space behind the oil ring in the ring groove is large, then the volume of oil existing in the cavity space and the volume of oil rising from the oil ring are also become large, whereby the oil consumption also consequently increases.
Blow-by gas has an affect of blowing the oil in the cavity space behind the oil ring down toward the oil drain holes. However, because the cavity space is large, and the opening of the oil drain holes is positioned lower than the coil expander, the blowing-down effect of blow-by gas is restrained, thereby leaving some oil on the upper side of the cavity space.
The present invention is made to prevent the increased oil consumption due to the cavity space behind the oil ring in the ring groove. An object of the present invention is therefore to provide a ring groove for two-piece oil ring, capable of reducing the oil consumption.
To accomplish the object above, according to the present invention, a ring groove (11), of a piston (20), for a two-piece oil ring (10) is provided that has a ring body (5) equipped with a coil expander (6), where the ring body has an upper and a lower outer surfaces, both orthogonal to the axial direction of the piston. The ring groove has an upper inner surface (13) and a lower inner surface (14), both orthogonal to the axial direction of the piston; a radially inward-most, bottom surface (12) connecting the upper and lower inner surfaces; and oil drain holes (18) each having an opening (19) thereof located on the bottom surface. The ring groove has an axial width slightly greater than the width of the two-piece oil ring.
According to an embodiment of the present invention, the ring groove has the radially inward ends (15, 16) of the upper and lower inner surfaces of the ring groove respectively located radially inwardly apart by a first predetermined distance from the radially inward ends of the upper and lower outer surfaces of the ring body when the oil ring is mounted within the ring groove so that the outer circumferential surface of the ring body is in contact with the inner circumferential surface of a cylinder (7) engaged with the piston, where, according to an aspect of the present invention, the first predetermined distance is preferably not smaller than {fraction (1/300)}, and not greater than {fraction (1/100)}of the inner diameter of the cylinder (7).
The bottom surface (12) of the ring groove, according to the embodiment, comprises: a concave surface line formed by a generating line (17, 21) drawn on a cross-sectional plane of the ring groove along the centerline of the piston, the generating line having the upper and lower ends thereof located coinciding with the radially inward ends (15, 16) of the upper and lower inner surfaces of the ring groove, the generating line surrounding the coil expander (6) projecting from the radially inward end of the ring body so that the generating line is apart from the surface of the projecting portion of the coil expander by a certain predetermined distance. According to an aspect of the present invention, the certain predetermined distance is preferably around a range of {fraction (1/300)} to {fraction (1/100)} of the inner diameter of the cylinder (7).
Although the ring groove, according to the present invention, has the same axial width and radial depth as with the conventional ring grooves, the ring groove, according to the present invention, has a by far smaller cavity space behind the oil ring, or ring body, than with the conventional ring grooves. Thus, the oil volume remaining in the cavity space to flow out to the gap above the upper surface of the ring body, is remarkably reduced in comparison with the conventional ring grooves.
According to another embodiment of the present invention, the opening of the oil drain hole is located approximately in the center of the axial width of the ring groove in the bottom surface of the ring groove, which enables the oil remaining in the cavity space to be returned to the oil pan by utilizing the blowing-down effect of the blow-by gas.
In case of certain difficulty in utilizing the blowing-down effect, however, it may be arranged, similar to the conventional ring grooves, that the opening of the oil drain hole is located as low as the lower inner surface of the ring groove, or even lower, to enable the oil scraped down by the oil ring to be quickly returned to the oil pan.
Thus, with the arrangement described above, according to the present invention, a ring groove is obtained that has an advantage in that the oil volume remaining in the cavity space to flow out to the gap above the upper surface of the ring body is remarkably reduced in comparison with the conventional ring grooves, thereby enabling to reduce the oil consumption outstandingly.